A magnetoresistive random access memory is a memory that is typically fabricated from anisotropic magnetoresistance materials or from giant magnetoresistance materials to form a plurality of memory cells. The cells of early magnetoresistive random access memories, such as anisotropic magnetoresistive memories, used magnetic hysteresis to store data, and the cells were read using the cells' magnetoresistance. Such cells could be integrated on an integrated circuit chip. A cell of such a memory typically comprised two ferromagnetic layers sandwiching a conductor.
Giant magnetoresistance materials permit higher signals and faster read access time as compared to anisotropic magnetoresistive materials. A pseudo spin valve cell can be formed by sandwiching an appropriate electrically conducting non-magnetic layer between two magnetic films. This cell shows a greater change in magnetoresistance than that achieved with the anisotropic magnetoresistive effect.
The pseudo spin valve (PSV) cell supports the operation of a magnetoresistive random access memory (MRAM). A pseudo spin valve typically has two magnetic layers that sandwich a non-magnetic conductive layer therebetween. The two magnetic layers are designed so that one layer tends to switch at lower fields than the other. The difference in switching fields may be obtained, for example, by using two magnetic films of the same material, but with different thicknesses. In this case, the thinner film switches at lower fields, and the thicker film switches at higher fields. The resistance of the pseudo spin valve is lowest at fields where the magnetization of the thicker film is aligned in the direction of the magnetization of the thinner film. The thinner film is used to read the storage state that is stored in the thicker film.
In the pseudo spin valve structure, the magnetization of the two magnetic layers can be predominantly anti-parallel or parallel, making the resistance of the film high or low, respectively. The stored bit is read non-destructively by applying a sense current together with a negative and then a positive word current. For read, the word current magnitude is chosen such that the magnetic field generated by the word current in the presence of the sense current is enough to switch the thinner magnetic layer, but not enough to switch the thicker magnetic layer used to store data. For write, the word current magnitude is chosen such that the magnetic field generated by the word and digit and/or sense currents switches the addressed, selected thicker magnetic layer without disturbing the other memory cells.
FIG. 1 shows the successful write characteristic of a pseudo spin valve, and FIG. 2 shows the successful read characteristic of a pseudo spin valve. FIG. 3 shows the write cycling error that appears as a read disturbance error in which the thicker storage magnetic film is switched prematurely at the selected write current under selected-device write conditions. A biased remnant magnetization, for example, which can result from biasing from the digit and/or sense current, can enable selected write switching at the selected write switching field during a read cycle. Accordingly, the change in resistance dR of the pseudo spin valve can be reduced, for example, by 50%. This reduction in dR can occur, for example, because the sustained rotated magnetization initiated and/or sustained in the device, including the bit ends, can allow the magnetization between the thin and thick layers to be not fully parallel and/or anti-parallel. Also, switching asymmetry and variability can be caused by a bias resulting from remnant magnetization states in the bit induced by the vector sum of the digit, sense, and/or word fields. Undesired switching of the magnetization can result from less than optimal bit ends of the individual storage pseudo spin valve devices, particularly in instances when magnetization reversal is initiated in bit ends.
The present invention is directed to a magnetoresistive device such as a magnetoresistive memory device, an anisotropic magnetoresistive memory device, a giant magnetoresistive memory device, a pseudo spin valve memory device, a spin valve memory valve, etc. having end shapes that mitigate or eliminate one or more of these or other problems.